Recently, wireless communication devices are using a co-channel transmission mode, in which different pieces of information are sent on two polarization signals that have the same frequency and different phases. The two polarization signals are referred to as a vertical polarization signal and a horizontal polarization signal and are sometimes referred to as V polarization signal and an H polarization signal. Interference may occur between the V polarization signal and the H polarization signal, which is referred to as orthogonal cross-polarization interference or cross-polarization interference. Accordingly, the wireless communication devices that use a co-channel transmission mode are provided with an orthogonal cross-polarization interference compensator, which compensates for orthogonal cross-polarization interference.
In order to compensate for orthogonal cross-polarization interference by using the orthogonal cross-polarization interference compensator, it is required to synchronize other polarization components (i.e., interfering components), which interfere with an own polarization, with carrier components (i.e., a carrier frequency) of the other polarization components, which are being input to the orthogonal cross-polarization interference compensator as a reference signal. In the case of synchronizing these components, receive local synchronization in which receiving local signals are synchronized to respective polarization signals is appropriate in sub-synchronous detection mode.
Receive local synchronization includes local division and reference synchronization. In local division, one output signal from a RF oscillator is divided into two signals, each of which is a local signal of a respective polarization signal. In addition, in reference synchronization, local oscillators are provided for respective polarization signals, and output signals from the local oscillators are used as local signals of the polarization signals, respectively, after being synchronized to one reference signal.
In local division, since the local signals of the polarization signals are the same signal, the other polarization signal component, which interferes with the own polarization signal, is completely synchronous with the carrier component (i.e., the reference signal) of the other polarization signal, which is input to the orthogonal cross-polarization interference compensator. Thus, phase noise of the local oscillator does not influence the compensation characteristics of the orthogonal cross-polarization interference compensator. However, local division is disadvantageous in terms of the reliability of the communication path since both polarization signals are disconnected if the local oscillator is broken.
In reference synchronization, even if one oscillator is broken, one communication path is ensured when the other oscillator is operating. In this mode there is improved in the reliability of communication path over local division. In addition, in reference synchronization, since the output signals of the local oscillators are synchronous to one reference signal, the frequency of the other polarization signal is consistent with that of the own polarization signal.
However, since phase noise from one oscillator is not related to that from the other oscillator, a phase difference originating from phase noise occurs between the other polarization component, which interferes with its own polarization, and the carrier component of the other polarization component, which is input to the orthogonal cross-polarization interference compensator. When the phase difference or a changing rate of the phase difference increases, the input/output characteristics of the orthogonal cross-polarization interference compensator degrade.
A phase corrector capable of suppressing the phase difference originating from phase noise is phase corrector 113 as shown in FIG. 1. Phase corrector 113 detects a phase difference between a local signal of own polarization input and a local signal of other polarization input based on an error signal, which is obtained from the result of demodulating an output signal of orthogonal cross-polarization interference compensator 112 and a main signal (i.e., an own polarization input). In addition, phase corrector 113 suppresses the phase difference originating from phase noise by adjusting the phase of the output signal of orthogonal cross-polarization interference compensator 12 based on the phase difference.
This technology is disclosed in Patent Document 1.
Patent Document 1: Japanese Patent Publication No. 2000-165339